1. Field of the Present Invention
The present invention relates to a white light-emitting structure, in particular to a multi-wavelength white light-emitting structure for generating good color rendering.
2. Description of the Related Art
Technological developments with light emitting diodes (LEDs), such as the improvement of their quality, efficiency, life and color rendering, have led to LEDs being all around us—they are commonly applied in, for example, traffic signs, cell phones, Christmas ornaments and lighting apparatuses, etc. Currently, a white light-emitting structure cannot be produced by directly using a light emitting diode chip that emits white light. The general method uses a light emitting diode chip that emits color light and excites a phosphor having a specified wavelength. These two wavelength lights are then mixed to emit white light.
To produce a white light-emitting structure, one method combines a red light emitting diode chip, a green light emitting diode chip and a blue light emitting diode chip with a predetermined ratio. Another method uses a blue light emitting diode chip or a UV light emitting diode chip and a specified phosphor.
FIG. 1 shows U.S. Pat. No. 5,998,925, titled “Light Emitting Device Having A Nitride Compound Semiconductor And A Phosphor Containing A Garnet Fluorescent Material.” The present invention uses a nitride compound semiconductor to emit blue light and excites a phosphor containing a garnet fluorescent material to emit yellow light. The blue light and the yellow light are then mixed to emit white light. The light emitting device comprises a phosphor 10a, a light emitting element 11a, a connecting wire 12a, a protection mask 13a and two conducting-wire pins 14a and 15a. 
TW publication patent No. 200417064, titled “A Method For Producing A White light-emitting structure Source Excited By A UV Light” is another prior art. A white light is produced by using a yellow-white phosphor that has a flat-broad wavelength band and is excited by a UV light emitting diode chip (with a wavelength of between 380˜400 nm), and a blue light emitting diode chip.
The white light-emitting structure of the prior art uses a blue light emitting diode to excite a yellow phosphor and generates a yellow light. The yellow light and the blue light are then mixed to emit high luminance white light. When the white light-emitting structure is used as a lighting apparatus any objects upon which it shines will have a yellow or blue tint.
In order to improve the color rendering, the prior art adopted a UV light emitting diode to excite a red phosphor, a green phosphor and a blue phosphor. When we select the phosphors with an appropriate ratio to emit white light, it is necessary to meet a condition—the exciting light must be absorbable by these phosphors and the absorbability index of the phosphors must be similar. This requirement increases the difficulty of choosing suitable phosphors.